Band-pass coupling network



Feb. 6, 1951 w. H. FORSTER 2,540,817

BAND-PASS COUPLING NETWORK Filed Jan. 30, 1947 INVENTOR. W/LLMM-H. P031727? Patented Feb. 6, 1951 UNITED STATES PATENT OFFICE BAND-PASS COUPLING NETWORK William H. Forster, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pal, a corporation of'llennsylvania Application January 30, 194?,Serial No. 725,203

3 Claims. (c1. 179-171) The invention herein described and claimed relates to improvedmeans for couplingtogether a single sided electrical circuit and a double-sided or push-pull electrical circuit. (The terms single-sided and, double-sided are herein used to refer to circuits which are sometimes called single-ended and double-ended circuits respectively, but the former terms will be used throughout thisspecification.)

The improved means provided by the present invention is adapted to transfer energy in either direction, i. e., the invention contemplates the provision of means adapted to couple a singlesided output circuit to a push-pull inp-utcircuit, or to couple a pushepull output circuit to a singlesided input circuit. The present invention may be utilized to particular advantage in radio amplifier circuits, involving conversion of single-sided signals to push-pull signals and/or of push-pull signals to single-sided signals, where it is desired. to amplify, substantially uniformly, a relatively wide band of very high frequencies. The term wide band is here used to mean a band whose width is of the order of per cent or more of the nominal'carrier frequency.

In very-high-frequency amplifiers of the above types, uniform gain over a wide band is dlfficult toachieve due to the presence of unwanted distributed capacitances and stray inductances.

The magnitude of the distributed capacitances unavoidably present in very-high-frequency circuits frequently precludes the use of capacitance tuning and compels the employment of inductance tuning. But the mere employmentof inductance tuning doesv not, insure that adequate gain will be realized since the stray inductances present in the circuit may be large enough tov prevent the realization of adequate mutual coupling between stages. For example, if transformer coupling and inductance tuning be employed in a very-high-frequency amplifier involving conversion of single-sided signals to pushpull signals, adequate gain over a wide band is extremely difiicult to achieve due to the inability to ellect adequate mutual coupling.

I have discovered that substantial improvement in gain is realized over prior art very-high-frequency circuits by coupling the single-sided and push-pull circuits together by a network which includes, a pair of reactances of opposite. sign connected in amanner to be described. The novel coupling arrangement is also employable at lower frequencies, but the improvement realizable at. the lower frequencies over' prior art circuits isless than at the higher frequencies.

His an object of this invention to provide improved means for coupling together a singlesided electrical circuit and a double-sided or push-pull circuit.

It isanother object of. this invention to provide means adapted to effect improved coupling between a-single-sided circuit and a push-pull circuit over a wide band of very-high frequencies. It is one of the specific objects of this invention to provide an improved, very-high-frequency, wide-band; band-pass amplifier stage adapted to convert a. single-sided input signal into a push-pull outputsignal. Such amplifier circuits are frequently referredto as paraphase amplifiers.

Another specific object of thisv invention is to provide animproved, very-high-frequency, wideband, band-pass amplifier stage-adapted to convert a push-pull input signal into a single-sided output signal;

'It is another objectof this invention to provide relatively high-impedance coupling means between. a single-sided'tank circuit and a push.- pull tank circuit, thus reducing the deleterious effectxof'lead inductance and lead capacitance. These and other objects, advantages and features. of thepresent invention will become clear from the following detailed description of the specificembodiments which are illustrated in the accompanying. drawings, wherein:

Figure 1' is a schematic representation of a paraphase amplifier which includes a preferred embodiment of coupling network adapted toconvert a single-sided input, signal into. a push-pull output signal; and

Figure 2 is a schematic representation of a coupling network adapted to ccnverta. push-pull input signal into. apsingle-sided output signal.

Referring to. Figure 1', there is shown a paraphase amplifier circuit comprising a source of singlessided signal Which is. applied to the control, grid of pentode l0. Thesingle-sided output signal of tube i0 is converted to a push-pull signal by my novel coupling network 35. andfthe push-pull signalv is applied. to the control grids of a pair of pentodes I]! and I2. The single-sided output circuit: of tube i,ii;,.Whicl1 also forms the single-sided input, portion of coupling networks 35... includes tank circuitl3 comprised of variable inductance fitand capacitance {5. Variable inductance I4 is. connected between plate lead is and a source of plate potential, 3+. Capacitance is may comprise the inherent distributed representation.

.capacitances is and 29.

The push-pull output portion of coupling network 35 comprises a tank circuit I? which includes a number of reactances common to the input circuits of both of the push-pull tubes. These reactances include variable inductance l8, capacitance is and capacitance 29. Variable inductance i8 is ungrounded and is connected between control grid 23 of tube H and control grid 2c of tube E2. The omission of a ground connection from inductance i8 is deemed important to the satisfactory operation of the coupling network as will be discussed more fully later. Capacitance l9 may comprise the inherent distributed input capacitance of pentode H, and

capacitance may comprise the inherent distributed input capacitance of pentode l2 shunted, if desired, by a fixed or variable capacitor for a purpose subsequently explained.

Resistors 2i and 22, connecting grids 23 and 2d respectively to ground, are damping resistors which tend in known manner to flatten the frequency-response characteristic of the stage over the band of operating frequencies. The damping resistance is preferably placed in the input circuits of the push-pull tubes only, but, if desired, damping resistance may also, or alternatively, be connected in shunt with the single-sided output circuit 13 of pentode Iii.

The single-sided output circuit :3 of pentode Iii is susceptance-coupled to the push-pull output portion ll of the coupling network 35 by means of a coupling capacitor 25 and a coupling inductance 2t. (Capacitor 217 is merelya blocking capacitor intended to isolate grid 24 from the high anode-voltage, 25+.) Coupling capacitor 25 is connected between a high potential point on output circuit l3 and one end of variable inductance 18, while coupling inductance 26 is connected between the said high potential point and the opposite end of variable inductance IS. in Figure 1, coupling capacitor 25 is shown connected to the upper end of inductance l8, and coupling inductance 2c is shown connected to the lower end of inductance 13, but these coupling elements may be reversed in posiiton, if desired.

For best results, capacitance 25 and inductance 26 are of such value that, at the center frequency of the pass-band, the reactance of capacitor 25 is substantially equal to that of inductance 25. The optimum value of each of these coupling reactances at frequencies within the pass-band is dependent upon the desired width of the pass-band. For wider pass-bands, the coupling reactances are relatively smaller, 1. e. the coupling is closer. In many cases, as for example in a case involving a pass-band ofthe order of 20 per cent of a nominal frequency of the order of 100 megacycles, the value of each of the coupling reactances will be larger than the reactance of the output capacitance l of Figure 1 or of either of the input capacitances I9 or 20.

Variable inductance I3 is of such size that its reactance at the center frequency of the pass band' may be so adjusted as to be substantially equal to the series-combined reactances of input Resistances 2| and 22 may be of equal magnitude; or if more nearly similar frequency response is desired at grids 23 and 2s, resistance 22, on the inductancecoupled side of double-sided circuit ll, may be made larger than resistance 21 in order to compensate for the fact that the Q of the capacitance-coupled side of double-sided circuit I! will ordinarily be higher than that of the inductancecoupled side. The magnitudes selected for the damping resistances are, of course, dependent upon the amount of flattening desired for the frequency-response characteristic.

The output signals of push-pull tubes H and 42 may be combined in any known and suitable manner. In the circuit shown in Figure 1, the output signals of push-pull tubes H and 52 are applied to a tank circuit 23 comprised of several to 3+ by way of radio-frequency choke 32. The

impedance of choke 32 at the operating fre-' quencies is very high so that inductance 29 may be considered ungrounded, at radio frequency. Hence, it is not essential that the tap to 3+ be precisely from the center of the coil 29. Capacitances 353' and 38 may comprise the inherent distributed output capacitances of the plate circuits of tubes II and I2 respectively, and are shown in Figure 1 by dotted line representations. Capacitances 33 and 34 are merely coupling capacitors.

In practicing the invention, a coupling network was built in accordance with the embodiment depicted in Figure l, and used in the preamplifier circuits of a wide-band television relay system. In this system, the coupling network is preceded by a cascade of single-sided amplifiers and is followed by a push-pull power amplifier of several stages. The pass-band of the system is from to 125 mc., i. e. the system has a pass-band of 10 me. on each side of a center frequency of me.

In the system referred to above, the tubes which correspond to tubes W, H and [2 of Figure 1 are pentodes, known commercially as type 6AK5. The ontput capacitances of these tubes, which correspond to capacitance-s i5, 30 and 3| of Figure 1, are of the order of 4.5 ,upf. each. The input capacitance of tube ll, and the input capacitance of tube l2, represented in Figure 1 by capacitors I9 and 29, are of the order of 5.5 ,upf. each. The variable inductances, corresponding to inductances It and [3 shown in Figure 1 are comprised of #29 wire closely wound on onequarter inch forms. Inductance M in the singlesided output circuit of tube i5 is comprised of seven and one-quarter turns, and inductance I'B in the push-pull output circuit of coupling network 35 comprises eight and one-quarter turns. Coupling capacitor 25 which, in combination with inductance 26, couples the single-sided input portion of network 35 to the push-pull output portion of the network, has a value of approximately 1.5 ,U.,U.f., while coupling inductance 26 has a value of approximately 1.3 h. The capacitance of blocking capacitor 27 is 250 ,c f.

In determining the desired values of the coupling reactances, as for example, the values of elements 25 and 26 of'Figure 1, it is convenient for purposes of making the calculations, to treat the elements as susceptances rather than reactances, and for that reason, I frequently think of my improved coupling as susceptance coupling.

Referring now to Figure 2, there is shown a stage in which the coupling network of the present invention is employed to couple a push-pull circuit to a single-sided circuit. In Figure 2, the output signals of tubes 58 and 5| are applied in push-pull to a double-sided tank circuit 52 comtank circuit 52' is coupled to'the singlesided input circuit of tube tlby means of coupling inductance 58 and coupling capacitancebt; Tank circuit lit, which forms the sing'le sided output cir= cuit of the coupling network, aswell asthesinglee sided inputcir'cuitof tube is'co'm'prised of variable inductance 6"! and capacitance 62. In the circuit depicted'inFigureZ, capacitance '62 represents the inherent distributed inputjcap'acitance of tube For best resultscoupling elementsfiil and 559 are, of such value'ithat, at the center frequency of'the pass ba'nd, the reactance of inductance 58 is substantially equal to the reactance of capacitance '59, The optimum value of each of these coupling reac'tancesat frequencies within the pass-band is dependentupon the desired widthoi the pass band. For wider pass bands, the coupling reactances are relatively smaller, i. e. the suscepta'nces are larger and'the coupling is closer. ln'many case'sinvolving high frequencies and inductance tuning, the v'alueo'f each of the coupling reactances 5B and 59 is larger than, or at least as large as, the, reactance of either of the output capacitances 54 or 55, or of input capacitance 621 In practicing the invention, an amplifier stage was built substantially in accordance with the embodiment depicted inFigure 2, and used as a part of the power amplifier of the same wideband television relay system mentioned previously in connection with Figure 1; The power amplifier referred to comprises two push-pull stages, the second of which comprises the left hand portion of Figure 2. And as shown in Figure 2, coupling from push-pull to a single-sided circuit is accomplished by meanso'f my improved coupling network. a

The power amplifier referred to above has a pass-band of mc., extendin from 105 to 125 Inc. The second stage of this amplifier comprises i a dual pentode, known commercially as type 829B; this dual-unit tube corresponds to pentodes 5c and 5! of Figure 2. The output capacitance of each unit of this tube is of the order of 7.0 ,U/Lf. and is represented in Figure 2 by capacitances 5t and 55. Variable inductance 53 comprises seven turns of' #23 wire and has a center-tap v llCh may, or may not, be at the midpoint. Choke 5% offers a very high impedance to frequencies within the. p'ass-band's'o that inductance 53 maybe considered tobe ungrounded at the operating frequency. Coupling inductance 53 has a value of approximately 0.32 ,uh., while coupling capacitance 59 has a value of approximately 6 ,Lt/tf. The capacitance of blocking c'apacitor 63 is 250 [H.Lf.

Tube 5"! may be any tube suitable for the particular application. In the specific application being described, tube 51 is comprised of a cavity resonator, housing a pair of lighthouse triodes, type 2639, whose grids are driven inparall'el. The totalinput capacitance of these tubes, represented in Figure 2 bycapacitance 62, is o'f'the order of 55 t. Inductance, corresponding to inductance E! of Figure 2, is within the cavity resonator and is resonated with the grid capacitance. a V a I have described the structure of my improved coupling network and IQhave set rerun certain details regarding" circuits which have beenactu- 6 ally'built and used in accordance with my invention. The gain and frequency response of the circuits which are in use are very satisiactory and represent definite improvements over prior art circuits. The frequency response obtained over the pass-band extending from to m. is substantially symmetrical having'two peaksof substantially equal value with a dip-response therebetween.

In operation, the network is the equivalent of a pair of double-tuned circuits arranged in parallel, as will now be described. The single-sided tank circuit, as for example, circuit l3 of Figure 1, although physicall a unitary circuit, may be considered, for purposes of theoretical analysis, to be comprised of a pair of tank circuits in parallel, each having an inductive branch of twice the magnitude of inductance l4 and each having a capacitive branch of one-half the magnitude of capacitance l5.

Similarly, double-sided tank circuit ll of Figure 1 may be considered to be comprised of two tank circuits, i. e., an upper circuit comprising capacitance l9 and the upper portion of inductance l8, and a lower circuit comprising capacitance 20 and the lower portion or inductance l8. However, the said upper and lower portions of inductance H? are not equal except perhaps at one frequency within the pass-bandthat is, the electrical center of inductance I8 varies as the frequency varies; and the location of the electrical center is affected by the presence of coupling capacitance 25 in the upper circuit and of coupling inductance 26 in the lower circuit.

It is seen then that coupling network 35 of'Figure 1 functions as apair of double-tuned circuits connected in parallel, one of the double-tuned circuits being comprised of a fictional one-half of single-sided tank circuit I3 and the upper portion of double-sided tank circuit ll coupled to gather by capacitive reactance 25, and the other double-tuned circuit comprising the other onehalf of single-sided tank circuit l3 and the lower portion of double-sided tank circuit l'l' coupled together by inductive reactance 26. V

a The input impedances of both of the above double-tuned circuits have substantially the same frequency response. The transfer impedances which determine the signals on control grids 23 and 2d are substantiall apart. For wideband operation, the magnitudes of the transfer impedances may be unequal at most frequencies within the pass-band due to the fact that the total capacitance of each of the fictional doubletuned circuits may be unequal; but the phases will always be substantially 180 apart. If desired, the transfer impede-noes may be made substantially equal by suitable compensating means, as by adding capacitance to the side having the smaller capacitance. For example, capacitance is of Figure 1 may be made larger than capacitance I9, as by placing the leads of control grid 24 closer to chassis, or by the addition of a fixed or variable capacitor between controlgrid 24 and ground. Resistances 2i and 22 should in general be chosen to give equal Gs to the two halves of circuit [1.

It is not disadvantageous, however, to permit the transfer impedances to remain somewhat unviously, in which the pass-band is of the order of 17.3% of the nominal carrier frequency, the

amplitude unbalances in the stage corresponding to Figure 1 are of the order of 35%; and the unbalance was substantially constant at all frequencies within the pass-band. It was not deemed necessary to balance the transfer impedances to achieve amplitude balance in the stage referred to since neither of the push-pull tubes is overloaded. The percentage of amplitude unbalance will vary, of course, with the width of the passband; in general, the amplitude unbalancewill be network 35 would be comprised of two separate approximately twice as large as th bandwidth,

both values being expressed in terms of percentage.

I have indicated above that the frequency-response characteristic of the circuit of Figure 1 is substantially symmetrica1 about the mean frequency. I have also indicated one manner in which amplitude unbalances may be reduced, if desired. At th sam time, I have made it clear that amplitude unbalance of a reasonable magnitude may exist without disturbing the satisfactory' operation of the circuit.

In many cases, the overall system will require the conversion ofsingle-sided signals to push-pull and the subsequent reconversion or recombining of the push-pull signals to single-sided signals. In such cases, improved performance may be realized by reversing the positions of the coupling reactances in the two conversion stages. To i1- lustrate this point, assume that the push-pull output signals of the circuit of Figure l are applied to the input of the circuit of Figure 2, either directly or by way of intervening circuits. Assume further that a relatively small but noticeable amplitud unbalance obtains in the circuit of Figure 1 such that the signals on grid 2d of tube [2 are somewhat greater in magnitude than the signals on grid 23 of tube ll. If this be the case, then the signals applied to push-pull tubes 5il, M of Figure 2 will be of unequal amplitude.

ground.

It is known that in the design of push-pull amplifiers it is desirable to have equal signals on the plates of the push-pull tubes in order that the optimum plate suppl voltage may be employed. In the system being discussed equal or balanced plate signals may be achieved even though the input signals to the push-pull tubes be of unequal magnitude. Thi may be accomplished by arranging the coupling reactances 58, 53 in a reverse manner from that employed in Figure 1. That is to say, if in Figure 1 the source of singlesided signals be coupled capacitivel to the upper side of the push-pull circuit, then the upper side of push-pull output circuit of Figure 2 is preferably coupled inductively to the single-sided output circuit of Figure 2; and the lower side of the push-pull circuit, which is inductively coupled in Figure 1 to the single-sided source, is capacitively coupled in Figure 2 to the single-sided output circuit. Thus arranged, tube 5! of Figure 2, upon whose grid signals of larger amplitude have been assumed to be impressed, works into a load circuit of slightly lower impedance than does tube 50; hence tube 5! provides slightly lower gain than does tube 56, thus tending to compensat for the slight unbalance in the amplitudes of the applied signals.

Referring again to Figure 1, it has been stated previously that the omission of a ground connection from inductance is is important to the optimum operation of the circuit shown. There are several reasons why it is not desirable to return the physical center of inductance l8 to In the first place, if inductance l8 were grounded the output portion of coupling tank circuits, each tuned to resonance by one ofthe grid-input capacitances, and the amplitude unbalances on the grids of push-pull tubes II and I2 would vary widely at difierent frequencies within the pass-band, whereas with inductance I3 ungrounded it has been found that the amplitude unbalances remain substantially constant over the pass-band. A second reason for not returning inductance E8 to ground is that the tuning of the circuit is simplified. Observe that with inductance [8 not grounded the tuning problem is reduced to that of tuning a double-tuned circuit involving inductances l4 and I8; but if inductance l8 be grounded the problem becomes that of tuning a triple-tuned circuit, a considerably more difiicult task. Moreover, with inductance I8- ungrounded there is no danger of mistuning tank circuit ll with respect to the upper and lower portions; whereas, if the upper and lower portions be separately tuned, the possibility of mistuning is ever present. A third reason is that with inductance i8 not grounded the relatively strong currents which circulate within tank circuit H are excluded from chassis, whereas, if inductor 18 be connected at its mid-point to chassis, circulating currents in each of the two tank circuits, thus formed, would pass through parts of the chassis setting up in said chassis R.F. voltages easily capable of effecting regeneration in combination with other stages mounted on the same chassis. Moreover, chassis impedances would be introduced in the tank circuits with resultant losses in gain.

Having described my invention, I claim:

1. A double-tuned coupling network adapted to pass a wide band of very high frequencies and to interconnect a single-sided circuit and a push-pull circuit, said interconnecting coupling network comprising; a single-sided first tuned circuit tuned by a first inductance to substantially the center frequency of the passband; a doubleesided second tuned circuit tuned by a second inductance to said substantially center frequency; a capacitive reactance coupling a high potential point of said single-sided tuned circuit and one side of said double-sided tuned circuit; and an inductive reactance having substantially the same magnitude at said center frequency as said capacitive reactance coupling said high potential point of said single-sided tuned circuit and the other side of said doublesided tuned circuit.

2. A double-tuned amplifier adapted to pass a wide band of very high frequencies, said amplifier comprising: a first amplifier tube; second and third tubes comprising a pair of push-pull amplifier tubes; a single-sided output tank circult for said first tube, said output tank circuit being tuned by a first inductance to substantially the center frequency of the passband; a double-sided input tank circuit for said push-pull tubes, said input tank circuit being tuned by a second inductance to said substantially center frequency; a capacitive reactance coupling a high potential point of said single-sided output tank circuit to one side of said double-sided input tank circuit; and an inductive reactance of substantially the same magnitude at said center frequency as said capacitive reactance coupling said high potential point of said singlesided output tank circuit to the other side of said double-sided input tank circuit.

3. A double-tuned amplifier adapted to pass a wide band of very high frequencies, said amplifier comprising: first and second tubes comprising a pair of push-pull amplifier tubes; a third amplifier tube; a double-sided output tank circuit for said push-pull tubes, said output tank circuit being tuned by a first inductance to substantially the center frequency of the passband; a single-sided input tank circuit for said third tube, said input tank circuit being tuned by a second inductance to said substantially center frequency; a capacitive reactance coupling one side of said double-sided output tank circuit to a high potential point of said single-sided input tank circuit; and an inductive reactance of substantially the same magnitude at said center frequency as said capacitive reactance coupling 10 the other side of said double-sided output tank circuit to said high potential point of said singlesided input tank circuit.

WILLIAM H. FORS'IER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 2,359,618 Byrne Oct. 3, 1944 Certificate of Correction Patent N 0. 2,540,817 February 6, 1951 WILLIAM H. FORSTER It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 47, for the Word networks read network; column 4, line 42, for ontput read output; column 6, line 66, for Gs read Qs;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 14th day of August, A. D. 1951.

THOMAS F. MURPHY,

Assistant Uommz'ssz'aner of Patents. 

